proxy: update documentation
proxy: update documentation

In particular, note that /system/proxy/use-same-proxy and
/system/proxy/http/enabled are not actually used.

https://bugzilla.gnome.org/show_bug.cgi?id=648237

Distrotech/gsettings-desktop-schemas,GNOME/gsettings-desktop-schemas,GNOME/gsettings-desktop-schemas,Distrotech/gsettings-desktop-schemas